marți, 6 martie 2012


A. Uranium in natureUranium has expressed this in some minerals known long before his time destination. In the sixteenth century the maintenance deposit of silver was stored in a sterile Jachymov black, shining at night in a blue-green light, causing awe or superstitious fear of oamenilor.Mineri frequently gets ill is a serious illness , incurable and hitherto unknown, which is known today as actinic disease.Uranium is a fairly widespread in nature (more prevalent than gold, silver and platinum), can be found in many rocks, the waters of the oceans and even living organisms. It is true that uranium is a fairly dispersed, but nevertheless, there are accumulations of mineralized form where uranium concentration reaches high enough, even decades procemte. Mean concentrations of uranium in deposits currently in maintenance but not more than 1-2 tenths of a percent.Uranium reserves around the world, calculated on the average content of 4 x 10-4%, is 1.3 x 1014 tons, that is 800 times bigger than the gold reserves and 40 times than the silver in the earth's crust . Uranium is also more widespread than mercury, bismuth and cadmium, commonly used in engineering.It is obvious that the 1.3 x 1014 t of uranium found in the earth's crust can not be extracted and used for nuclear purposes reserves so it is not real. Industry are now considered recoverable reserves of 3.2 x 106 tons, so about a hundred million times smaller. Due to rapid perfectioarii ore processing technologies, the concept of recoverable reserves is very unstable not only in terms of uranium raw materials, but also many other useful minerals.So now, applying technological processes to industrial scale is considered rational processing of uranium ores with a content above 0.1%. In many countries the study supports the possibility of recovery of uranium minerals with very low uranium content.Are ordinary uranium recovery from sea water where its concentration is about 4 x 106 g / l. In this direction, especially in Japan, have achieved remarkable results even in semi-industrial scale. Thus, the actual recoverable uranium reserves on earth still not known today, but can appreciate as much higher than officially reported reserves.
The nearly 200 uranium minerals that were formed as a result of geo-chemical processes can be divided by their composition, into three groups:-Oxides-Combinations hydrated-SilicatesThe main primary uranium minerals are uranium oxides and pechblenda simplified - from which we obtain most of the uranium mined in the world.Uraninitul is uranium oxide UO2 tretavalent having formula. Oxygen content in this mineral is varied (generally greater than the corresponding stoichiometric chemical formula). With increasing the proportion of oxygen in uraninit, it changes color, from black becomes gray, hardness and lower density instead increases its solubility .Besides uranium and other elements uraninitul contains the most important of which are rare earth elements and thorium group. This gives them some technology features uraninitului bad given that rare-earth impurities are harmful in terms of nuclear.Pechblenda other uranium mineral has the chemical formula U3O8 and is a mixture of quadrivalent and hexavalent uranium oxides. Here, the oxygen content is variable and almost never corresponds chemical formula U3O8. As with size uraninitului proportion of oxygen leads to increased solubility pechblendei.Chemical composition of ore pechblenda, has the chemical formula U3O8 and is a mixture of quadrivalent and hexavalent uranium oxides. Here, the oxygen content is variable and almost never corresponds chemical formula U3O8.Chemical composition of pechblenda mineral found in Earth's crust is very complex differing from uraninit in that group does not contain any rare-earth elements, thorium, but frequently contains no lead, iron, copper, calcium, nickel, cobalt.Natural uranium oxide group also includes, besides pechblenda uraninit and so-called black oxide of uranium. They are superior natural oxides with variable chemical composition. Their oxygen content is generally higher than the pechblendei, the attendant in deposits, but they are located in areas closer to the surface of the earth crust. In appearance, is in the form of fine powders and amorphous, black, having a low hardness and low density.Uranium minerals processing technology becomes even more advantageous with how their oxygen content increases due to increased solubility in chemical reagents.Of primary uranium minerals - uraninit and pechblenda - were formed, as a result of geo-chemical processes occurring especially in surface deposits, secondary minerals. Initially, the uranium and plechbenda uraninit were oxidized under the influence of oxygen and water, until the hexavalent form. In the presence of water have formed various hydroxides of uranium, which reacted with oxides of other metals that were in the same area. Therefore, secondary minerals are found in all the superficial deposits of pechblenda and uraninit. They generally have a lower economic importance.Of secondary uranium minerals are being exploited on an industrial scale Carnot (uranovanadatul potassium) and complex uranium minerals in which uranium is found associated with titanium, and niobium tantanul.Uranium is found in other secondary minerals in the form of isomorphic impurity, also in different minerals of organic origin whose processing has not been achieved in large scale so far, but soon, their recovery becomes necessary.
Two. Prinipalele properties of uranium
Uranium was discovered in the French Revolution, that in 1789, the German chemist Martin Klaproth Heinrick, but merit its separation of pure metal form belongs Peligot E. French chemist.Uranium ranks 92 of the Periodic System, with the highest atomic weight of natural elements.Uranium metal in fresh section, has a silvery white color that darkens as the oxidant action aerului.acest metal is 19.05 g/cm3 specific gravity (one of the densest metals) and melting temperature of 1133oC. It is important to note that pure uranium metal, up to a temperature of 662oC, is crystallized in rhombic crystal system. Above this temperature, the crystal structure changes with increasing specific volume. This is a critical disadvantage in using uranium metal in nuclear reactors, limiting their operating temperatures.Chemical properties of uranium are determined by the valence electronic configuration layers. Thus, uranium is found in various chemical combinations with positive values ​​of 2, 3, 4, 5 and 6. Have high stability but hexavalent combinations, they are using all valence electrons (substrates 5f3, 6d1 and 7s2).From thermodynamic point of view, the most stable chemical combinations are oxides of uranium. Precisely for this reason uranium found in nature as oxides.Uranium dioxide, UO2, one of the main oxides of uranium metal looks black, 10.82 g/cm3 density and melting point 2176oC.Basic character and dissolve only by oxidation.In nuclear technology, uranium dioxide plays an important role both as an intermediate product, as well as product finitr.Under the action of air, uranium dioxide is oxidized to U3O8 at a speed proportional to the temperature rise. This oxide charge also an important place in the processing technology of uranium materials.Uranium trioxide, UO3, orange is a substance with a density of 7.28 g/cm3, which is obtained by calcination at 400oC 3-uranyl nitrate, ammonium, etc. to diuranatului. Uranium trioxide anhydride acts as a contact with hot water and acid forms Urania.The most important chemical property of the uranium is in solution tend to form uranyl ion, UO22 +, most participating in formation of uranium compounds.Uranium halide can form in all levels of valence. The most important halides are fluoride uranium industry. Tetrafluorura uranium, UF4, anhydrous, is a green powder, hard solibila, melting temperature 960oC, which is used to obtain uranium metal and high fluoride.Hexaflorura uranium, UF6, is a white substance which melts at normal pressure but gaseous jump to 56oC. The main disadvantage of using this compound in the technology of nuclear fuel is its great hygroscopicity.Uranyl salts by treatment with alkaline hydroxides excess uranium can form Urania (the general formula Me2UO4) passing imedeat in diuranati (Me2U2O7) insoluble in water. Ammonium and sodium Diuranatii have great importance in obtaining nuclear fuel cycle.